Java Program For Insertion Sort In A Singly Linked List

Last Updated : 14 Apr, 2023

We have discussed Insertion Sort for arrays. In this article we are going to discuss Insertion Sort for linked list.
Below is a simple insertion sort algorithm for a linked list.

1) Create an empty sorted (or result) list.
2) Traverse the given list, do following for every node.
......a) Insert current node in sorted way in sorted or result list.
3) Change head of given linked list to head of sorted (or result) list.

The main step is (2.a) which has been covered in the post Sorted Insert for Singly Linked List Below is implementation of above algorithm:

Java

// Java program to sort link list
// using insertion sort
public class LinkedlistIS 
{
    node head;
    node sorted;
 
    class node 
    {
        int val;
        node next;
 
        public node(int val) 
        {
            this.val = val;
        }
    }
 
    void push(int val) 
    {
        // Allocate node 
        node newnode = new node(val);
 
        // Link the old list of the 
        // new node 
        newnode.next = head;
 
        // Move the head to point to 
        // the new node 
        head = newnode;
    }
 
    // Function to sort a singly linked list 
    // using insertion sort
    void insertionSort(node headref) 
    {
        // Initialize sorted linked list
        sorted = null;
        node current = headref;
 
        // Traverse the given linked list 
        // and insert every node to sorted
        while (current != null) 
        {
            // Store next for next iteration
            node next = current.next;
 
            // Insert current in sorted linked list
            sortedInsert(current);
 
            // Update current
            current = next;
        }
 
        // Update head_ref to point to 
        // sorted linked list
        head = sorted;
    }
 
    /* Function to insert a new_node in a list. 
       Note that this function expects a pointer 
       to head_ref as this can modify the head 
       of the input linked list (similar to push()) */
    void sortedInsert(node newnode) 
    {
        // Special case for the head end 
        if (sorted == null || 
            sorted.val >= newnode.val) 
        {
            newnode.next = sorted;
            sorted = newnode;
        }
        else
        {
            node current = sorted;
 
            /* Locate the node before the 
               point of insertion */
            while (current.next != null && 
                   current.next.val < newnode.val) 
            {
                current = current.next;
            }
            newnode.next = current.next;
            current.next = newnode;
        }
    }
 
    // Function to print linked list 
    void printlist(node head) 
    {
        while (head != null) 
        {
            System.out.print(head.val + " ");
            head = head.next;
        }
    }
     
    // Driver code
    public static void main(String[] args) 
    {
        LinkedlistIS list = new LinkedlistIS();
        list.push(5);
        list.push(20);
        list.push(4);
        list.push(3);
        list.push(30);
        System.out.println(
        "Linked List before Sorting..");
        list.printlist(list.head);
        list.insertionSort(list.head);
        System.out.println(
        "LinkedList After sorting");
        list.printlist(list.head);
    }
}
// This code is contributed by Rishabh Mahrsee

Javascript

class Node {
  constructor(val) {
    this.val = val;
    this.next = null;
  }
}
 
class LinkedListIS {
  constructor() {
    this.head = null;
    this.sorted = null;
  }
 
  push(val) {
    const newNode = new Node(val);
    newNode.next = this.head;
    this.head = newNode;
  }
 
  insertionSort(headRef) {
    this.sorted = null;
    let current = headRef;
 
    while (current !== null) {
      const next = current.next;
      this.sortedInsert(current);
      current = next;
    }
 
    this.head = this.sorted;
  }
 
  sortedInsert(newNode) {
    if (this.sorted === null || this.sorted.val >= newNode.val) {
      newNode.next = this.sorted;
      this.sorted = newNode;
    } else {
      let current = this.sorted;
 
      while (current.next !== null && current.next.val < newNode.val) {
        current = current.next;
      }
 
      newNode.next = current.next;
      current.next = newNode;
    }
  }
 
  printList(head) {
    while (head !== null) {
      console.log(head.val + " ");
      head = head.next;
    }
  }
}
 
const list = new LinkedListIS();
list.push(5);
list.push(20);
list.push(4);
list.push(3);
list.push(30);
console.log("Linked List before Sorting..");
list.printList(list.head);
list.insertionSort(list.head);
console.log("LinkedList After sorting");
list.printList(list.head);

Output:

Linked List before sorting
30  3  4  20  5
Linked List after sorting
3  4  5  20  30

Time Complexity: O(n²), in the worst case, we might have to traverse all nodes of the sorted list for inserting a node, and there are “n” such nodes.

Space Complexity: O(1), no extra space is required depending on the size of the input, thus it is constant.

Please refer complete article on Insertion Sort for Singly Linked List for more details!